1. Field of the Invention
This invention is in the field of magnetic erasing heads, and particularly involves the use of a plurality of heads which provide static magnetic fields through which the tape or other magnetic medium passes in the erasing process.
2. DESCRIPTION OF THE PRIOR ART
There are two types of methods for erasing residual magnetization for magnetic recording media such as a magnetic tape. One is alternating current demagnetization, and the other is static magnet erasing. In alternating current demagnetization, an alternating current flows through a coil wound on the magnetic erasing head. When the magnetic medium passes over a gap on this erasing head, it is magnetized up to its magnetic saturated condition. As it passes away from the gap, it is re-magnetized reversely and repeatedly. Eventually, the magnitude of magnetization reduces to a value approximating zero. This method is widely used in the field of magnetic recording and reproducing apparatus because the residual magnetization is completely erased. However, there are some difficulties with it because when high coercive force magnetic materials are used, the electric power consumed for the erasing head becomes high. Such a high power loss causes the core of the erasing head to become easily saturated with the result that the erasing effect is decreased.
The static magnetic erasing method involves the use of a strong static magnetic field provided by a permanent magnet or a coil wound on a core. The recording medium is magnetized from the state of residual magnetization to the saturated state. This method can be used for a relatively small magnetic recording and reproducing apparatus such as a tape recorder operated by a battery because of its simplicity. The difficulty of this method is that there is a tendency to distortion of the output signal at the reproducing stage.
An improvement of the static magnetic erasing method was proposed in the prior art. A permanent magnet was used in which the magnetized polarity was changed repeatedly and the magnitude was decreased toward zero along the scanning direction of the magnetic tape. This improved erasing head has an advantage similar to that of the alternating current method, but has its own disadvantage. The erasing head requires a large number of magnetic poles and it causes the length of the erasing head to be rather long. As a result, it is not suitable for use in a small magnetic recording apparatus.